1. Non-contact air-coupled velocity measurements in planar samples Qiang LIU 1,2,Bogdan PIWAKOWSKI 2,Zoubeir LAFHAJ 1 1. Laboratoire de Mécanique de Lille (LML, UMR CNRS 8107), Ecole Centrale de Lille 2. Institut d’Electronique, Microélectronique et Nanotechnologies (IEMN UMR 8520), Ecole Centrale de Lille The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015

2. I.Context and goal II.Contact methods III.Non-contact methods IV.Experimental results V.Conclusions OUTLINE 2 23/10/2015 The 13 th International Symposium on Nondestructive Characterization of Materials

3. 1.Necessity to mesure velocity. 2.Traditional methods are contact type. 3.We have developed a non-contact air-coupled system in order to: a)Avoid coupling medium; b)Avoid surface preparation; c)Enable the automation of measurement. Context and goal 3 The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015 Objective: To measure velocities by using non-contact air-coupled technique with good accuracy.

4. 4 d 1 st N th Δt 1. Pulse-echo: Contact methods Receiver d 1 st N th Transmitter 2. Through-transmission: Transmitter/Receiver The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015 Need of coupling meduim Need of planar contact surface

5. 5 Immersion method Receiver Transmitter d Receiver Water The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015

6. 6 Non-contact air-coupled method Receiver Transmitter d Receiver Air The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015 0.01%

7. Two problems: 7 The immersion solutions developed for water cannot be applied directly for air because: The velocity in air is unstable, so it is better not to use velocity in air for the computation of velocity in sample. The transmission coefficient is too small to allow the emitted energy penetrate into the sample, so it is necessary to apply techniques which improve the signal level. The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015

8. 8 « Chirp» signal ReceiverTransmitter A A tt Correlation T A The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015

9. Experimental setup 9 PC Generator: Generation of chirp; Memory card: Acquisition of signal; LabView: Correlation. Pre Amplifier Motor control Movements in x, y,z directions; Transmitter PC Amplifier Motor control Receiver Transmitter z x y Receiver The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015 Power Amplifier y z x

10. Calibration 10 Before calibration After calibration L L y y Transmitter Receiver Transmitter Receiver Beam axis The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015

11. 11 First step: Calculation of V air L1L1 L2L2 ΔL 12 The moving direction of receiver Transmitter Receiver t a1 y (1) The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015

12. 12 Second step: Calculation of V L Receiver 1 2 3 Transmitter The moving direction of receiver 1 2 Δt p t p1 d y L (2) (3) The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015

13. 13 Second step: Calculation of V L Receiver 1 2 3 Transmitter The moving direction of receiver 1 2 Δt p t p1 d y L (2) (3) The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015

14. 14 Third step : Calculation of V T d α β Δy Transmitter Receiver 1 2 Δy α c1 <α<α c2 y The incident angle should greater than the first critical angel and less than the second critical angle. Δt s t s1 t s2 (4) (5) (6) The 13 th International Symposium on Nondestructive Characterization of Materials 23/10/2015

15. 15 Validation with Plexiglas plate 23/10/2015 Comparing with the velocity obtained by contact method we find: 1. For longitudinal velocity, the ‘Multiple’ approach is more precise than the ‘Direct’ approach, because the velocity in air isn’t used here. As well the standard error of ‘Multiple’ approach is smaller. 2. For shear velocity, the results of ‘Multiple’ and ‘Shift’ approaches are also more precise than the ‘Direct’ approach. The 13 th International Symposium on Nondestructive Characterization of Materials

16. 16 Results obtained with porous material : mortar sample 23/10/2015 Our conclusions are confirmed with a mortal sample. Comparing with contact approach: 1.For longitudinal velocity, our ‘Multiple’ approach produces only 0.7% error while the ‘Direct’ approach produces 43.8% error. 2.For shear velocity, our ‘Multiple’ approach provides only 26.5% error, Shift’ approach provides 2.3% error white the ‘Direct’ approach produces 38.3% error. The 13 th International Symposium on Nondestructive Characterization of Materials

17. Conclusions 17 1.‘Direct’ approach, comes from immersion method, and commonly used in non-contact air-coupled technique gives always a high error when compares it with contact method. 2.‘Multiple’ approach for longitudinal velocity and ‘Shift’ approach for shear velocity provide higher accuracy which is comparable with the contact method. 3.Non-contact air-coupled method has great potential for the rapid measurements of velocity in ‘difficult’, porous, rough surface materials, such as concrete. 4.The proposed approach can be easily automated and used in high speed automatic NDT systems. 23/10/2015 The 13 th International Symposium on Nondestructive Characterization of Materials

18. Thank you for you attention! 23/10/2015 18